Sometimes understanding what's happening is a difficult thing to grasp, the exact figures can be different for different lines with different conditions, but the principal is the same. Let me try and explain this way using a simplified explanation - the Noise Margin is the amount of gap between the Noise and the Signal, when the margin is higher, at the moment, in your case at 9dB, there is less noise present, when it drops to 6dB, there is more noise there. If it were to goto to 0 the signal would be severely corrupted by the noise usually to the point of causing a loss of connection and a resync.
When noise is present, it isn't necessarily something that is equally distributed across all the frequencies used by the broadband signal. It could be more severe at some frequencies which will cause corruption of some bits of data, this is seen as errors. Error correction causes the corrupted data to be retransmitted and it's such retransmission that can cause some lag and lower throughput speed.
I hope that's given you a clearer picture.
So when you switch something on that's generating a lot of noise the noise margin goes down, in this case from 9 to 6dB (approx). Correctly designed and functioning equipment shouldn't generate as much noise as appears to be happening here. Once you known where it's coming from, then steps can be taken to suppress, filter or perhaps repair the offending item. Sometimes just re-arranging where all the connecting leads are in relation to other leads can reduce or eliminate a problem - but not always.
I hope that explanation helps.
